Method of fabricating a copper damascene structure

ABSTRACT

The present invention provides a method of removing copper overlaying a tantalum-based barrier layer during the fabrication of a copper damascene structure having a plurality of copper lines. The method includes providing a chemical-mechanical polishing slurry and polishing the copper layer using the slurry until the tantalum-based barrier layer is exposed. The slurry includes an oxidizing agent that releases free radicals and a non-chelating free radical quencher that retards the corrosion of said copper lines during chemical mechanical polishing. Preferred non-chelating free radical quenchers are ascorbic acid, thiamine, 2-propanol, and alkyl glycols. The present invention also provides copper damascene structures formed according to the method.

This is a division of application Ser. No. 09/692,729 filed Oct. 19,2000, now U.S. Pat. No. 6,508,953.

FIELD OF INVENTION

The present invention relates to a slurry for chemical-mechanicalpolishing copper damascene structures, and more particularly, to aslurry that retards copper line corrosion during the chemical-mechanicalpolishing of copper overlaying a tantalum-based barrier layer during thefabrication of copper damascene structures.

BACKGROUND OF THE INVENTION

Integrated circuits are made up of millions of active devices formed inor on a substrate such as silicon or gallium arsenide. The activedevices are typically isolated from each other using silicon-baseddielectric materials. The active devices are usually formed in multiplelayers that are interconnected to form functional circuits andcomponents. Interconnection of active devices is typically accomplishedthrough the use of well-known multilevel interconnection processes suchas the process disclosed in Chow et al., U.S. Pat. No. 4,789,648.

Copper is a highly preferred electrically conductive material for use infabricating integrated circuits because it has superior electromigrationresistance and lower resistivity than many other electrically conductivematerials such as aluminum. Copper wiring and interconnects allow forthe use of higher critical current in integrated circuits, which cangreatly improve the performance capabilities of such devices.

The use of copper in integrated circuits, however, does present somedifficult challenges. Copper readily diffuses into conventionalsilicon-based dielectric materials such as polysilicon,single-crystalline silicon, silicon dioxide, low-k inorganic and organicmaterials, and the like. Once these silicon-based materials have beencontaminated with copper atoms, the dielectric constant of thesilicon-based dielectrics is adversely affected. In addition, oncesemiconductive silicon-based materials are copper doped, transistorsmade within or in close proximity to the copper doped silicon-basedregions either cease to function properly or are significantly degradedin electrical performance. Therefore, a barrier layer or liner film mustbe applied to the silicon-based dielectric layer in order to preventcopper diffusion.

One of the presently preferred methods of fabricating integratedcircuits having copper wiring and interconnects, which are also known ascopper damascene structures, generally comprises providinginterconnected copper wiring or metallization patterns in discretelayers of dielectric film. The materials typically used to form thesedielectric film layers include phosphosilicate glass,borophosphosilicate glass, and silicon dioxide. The dielectric layer isetched or otherwise processed to pattern a series of trenches and/orholes therein. A thin barrier layer or liner film, generally not morethan approximately 300 Å thick is then deposited over the patterneddielectric layer. Preferred barrier layers or liner films comprise thinfilms of tantalum (Ta) or tantalum nitride (TaN) or both Ta and TaNdisposed over one another to form a Ta/TaN stack. Such liners arecommonly deposited by physical vapor deposition, which is also known assputter deposition, or they may be deposited by a chemical vapordeposition to form a more conformed coating. The Ta and/or TaN linercoats the surfaces of the trenches and holes as well as the uppersurface of the dielectric layer to prevent copper atom diffusion andalso to provide good adhesion between the copper layer and thedielectric layer. A layer of copper approximately 3,000-15,000 Å thickis then deposited over the liner layer so as to completely fill thetrenches and/or holes. The filled trenches thus form a network of copperlines whereas the filled holes form vias or interconnects. The finalstep in the process of fabricating an integrated circuit, which is alsoknown as the copper damascene process, is removing the copper layer andtantalum-based barrier layer from the upper surface of the dielectricfilm layer leaving only the copper filled trenches and holes. This istypically accomplished by chemical-mechanical polishing.

In a typical chemical-mechanical polishing process, the metallizedsurface of the copper damascene structure is placed in direct contactwith a rotating polishing pad at a controlled downward pressure. Achemically reactive solution commonly referred to as a “slurry” ispresent between the pad and the surface of the copper damascenestructure during polishing. The slurry initiates the polishing processby chemically reacting with the surface of the metal film beingpolished. The polishing process is facilitated by the rotationalmovement of the pad relative to the substrate and the presence of theslurry at the film/pad interface. Polishing is continued in this manneruntil the desired film or films are removed.

The composition of the slurry is an important factor in determining therate at which metal film layers are removed by chemical-mechanicalpolishing. If the chemical agents in the slurry are selected properly,the slurry can be tailored to provide effective polishing of specificfilm layers at desired polishing rates while minimizing the formation orcreation of surface imperfections or defects. In some circumstances, thepolishing slurry can preferably provide controlled polishingselectivities for one or more thin film materials relative to otherthin-film materials.

Prior art chemical-mechanical polishing slurries used to remove copperoverlaying tantalum-based barrier layers have exhibited a highselectivity for copper as compared to the tantalum-based materials. Thisadvantageously permits the rapid removal of the copper layer overlayingthe tantalum-based barrier layer. However, the aggressive chemicalaction of these prior art polishing slurries disadvantageously tends tocorrode the copper lines of the copper damascene structure duringpolishing, resulting in failure of the active devices or inconsistencyin their performance.

A need exists for an improved chemical-mechanical polishing slurry foruse in removing copper overlaying a tantalum-based barrier layer duringthe fabrication of a copper damascene structure. Such an improvedchemical-mechanical polishing slurry would preferably remove copperoverlaying tantalum-based barrier layers at a high enough rate to insureacceptable throughput while at the same time retarding the corrosion ofcopper lines.

SUMMARY OF INVENTION

The present invention provides a slurry for chemical-mechanicalpolishing copper damascene structures, and more particularly, to aslurry that retards copper line corrosion during the chemical-mechanicalpolishing of copper overlaying a tantalum-based barrier layer during thefabrication of copper damascene structures. The slurry according to theinvention comprises an oxidizing agent that releases free radicals and anon-chelating free radical quencher that is effective to retard thecorrosion of said copper lines during chemical-mechanical polishing. Theoxidizing agents that release free radicals used in the slurry accordingto the invention are preferably selected from the group consisting ofperoxides, peroxydiphosphates, and persulfates. The non-chelating freeradical quenchers used in the slurry according to the invention arepreferably selected from the group consisting of ascorbic acid,thiamine, 2-propanol, and alkyl glycols, with ascorbic acid being mostpreferred. The non-chelating free radical quenchers in thechemical-mechanical polishing slurry according to the inventionsurprisingly retard copper line corrosion during the polishing of copperdamascene structures without reducing the copper polishing rate tounacceptable levels. The anti-corrosion effect is independent of pH, butwhen the pH of the slurry is adjusted to from about 4.0 to about 7.0,the removal rate of copper is maximized.

The foregoing and other features of the invention are hereinafter morefully described and particularly pointed out in the claims, thefollowing description setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principles of the present inventionmay be employed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In co-pending application Ser. No. 09/277,454, the specification ofwhich is hereby incorporated by reference, it was disclosed that thegeneration of relatively large concentrations of free radicals cansubstantially increase the rate of copper removal duringchemical-mechanical polishing. An undesirable side-effect of thepresence of relatively large concentrations of free radicals duringchemical-mechanical polishing is copper line corrosion.

The term “corrosion” as used in the specification and in the appendedclaims is intended to refer to the phenomenon wherein usuallyirregular-shaped pits or depressions are chemically etched into thecopper lying in the trench or hole during the chemical-mechanicalpolishing of copper damascene structures. As used in the specificationand in the appended claims, the term “corrosion” is not intended torefer to the usually shallow depressions in the copper lying in thetrench or hole that can occur due to the chemical-mechanical action ofthe polishing pad, which is commonly known as “dishing.”

It is well-known in the art that benzotriazole (BTA) and compoundshaving similar functionality can be added to chemical-mechanicalpolishing slurries to protect copper lines from corrosion. As explainedin Sasaki et al., U.S. Pat. No. 5,770,095, copper reacts with BTA toform a secure film comprising a copper chelate compound or complex. Thecopper chelate film serves as a protective barrier film to preventoxidization or corrosion of the underlying copper film by the chemicalagents in the slurry. Unfortunately, when a small amount of BTA is used,it is very difficult to control the consistency of the slurryperformance. And, when a larger amount of BTA is used, the copperremoval rate is decreased to an unacceptably low level.

The present invention provides a chemical-mechanical polishing slurrythat can protect copper lines from corrosion without the need for BTA orother chelates. Applicants have surprisingly found that copper linecorrosion can be effectively suppressed by incorporating a non-chelatingfree radical quencher in a chemical-mechanical polishing slurry withoutreducing the copper polishing rate to unacceptable levels. Accordingly,the slurry according to the invention comprises an oxidizing agent thatreleases free radicals and a non-chelating free radical quencher that iseffective to retard the corrosion of said copper lines duringchemical-mechanical polishing.

The oxidizing agent used in the slurry according to the presentinvention releases free radicals during polishing. Suitable oxidizingagents for use in the invention include, for example, peroxides,peroxydiphosphates, persulfates, and combinations of the foregoing.Presently, the most preferred oxidizing agents for use in the slurryaccording to the invention are hydrogen peroxide, ammonium persulfate,and/or potassium persulfate. The oxidizing agent preferably comprisesfrom about 0.01 % to about 15.0% by weight of the slurry. Morepreferably, the oxidizing agent comprises from about 0.1% to about 10.0%by weight of the slurry. When hydrogen peroxide is used, the oxidizingagent optimally comprises from about 0.5% to about 5.0% by weight of theslurry.

The slurry according to the present invention also comprises at leastone non-chelating free radical quencher. As used in the specificationand in the appended claims, the term “non-chelating free radicalquencher” is intended to refer to a compound that does not readilychelate or otherwise complex with copper, but that is capable ofreacting with a free radical species to retard its reactivity and thusprotect the copper metal from corrosion. The presently most preferrednon-chelating free radical quencher for use in the slurry according tothe invention is ascorbic acid, which is also known as Vitamin C. Otherpreferred non-chelating free radical quenchers include thiamine(3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-5-(2-hydroxyethyl)-4-methylthiazoliumchloride), which is also known as Vitamin B₁, 2-propanol, and alkylglycols. It will be appreciated that derivatives and precursors of thesenon-chelating free radical quenchers can also be used in the invention,and that the appended claims should be read with sufficient breadth toencompass such derivatives and precursors.

The preferred amount of non-chelating free radical quencher used in theslurry is the smallest amount that is effective to retard the corrosionof said copper lines during chemical-mechanical polishing. Typically,the non-chelating free radical quencher will comprise from about 0.01%to about 5.0% by weight of the slurry. When ascorbic acid is used as thenon-chelating free radical quencher, an amount from about 0.1% to about1.0% by weight of the slurry is usually effective in retarding thecorrosion of copper lines.

Optionally, the slurry according to the present invention can furthercomprise abrasive particles. However, it will be appreciated that forsome applications it may be preferable for the slurry not to containabrasive particles. In such circumstances, the mechanical polishingaction is provided by the pressure of the polishing pad. When present,abrasive particles further assist in performing the function ofmechanical grinding.

Abrasive particles that may be used in the slurry according to thepresent invention may comprise any one or a mixture of a variety ofabrasive particles that are conventionally utilized inchemical-mechanical polishing slurries. Examples of suitable abrasiveparticles include alumina, ceria, copper oxide, diamond, iron oxide,nickel oxide, manganese oxide, silica, silicon carbide, silicon nitride,tin oxide, titania, titanium carbide, tungsten oxide, yttria, andzirconia, and combinations thereof. Presently, the preferred abrasivesare silica, alumina, and ceria, with alumina and silica being mostpreferred.

The abrasive particles preferably have a mean size ranging from about0.02 to about 1.0 micrometers, with a maximum size of less than about 10micrometers. It will be appreciated that while particle size is not perse critical, if the abrasive particles are too small, then the polishingrate of the slurry can be unacceptably low. It will also be appreciatedthat if, on the other hand, the particles are too large, thenunacceptable scratching can occur on the surface of the article beingpolished. The abrasive particles can be present in the slurry in anamount up to about 60% by weight of the slurry, more preferably fromabout 0.5% to about 30.0% by weight of the slurry, and optimally withinthe range of from about 3.0% to about 10.0% by weight of the slurry.

Preferably, the slurry according to the invention further comprises asolvent. The preferred solvent for use in the slurry according to theinvention is deionized water. Other solvents conventionally used inchemical-mechanical polishing slurries can also be used. The slurry mayalso contain optional surfactants, pH adjusters, pH buffers,anti-foaming agents, and dispersing agents, which are well known.

The anti-corrosion effect produced by incorporating a non-chelating freeradical quencher in the slurry according to the invention is not pHdependent. In other words, the corrosion retardant phenomenon isobserved throughout a broad range of pH. However, when the pH of theslurry is adjusted to from about 4.0 to about 7.0, the rate of desiredcopper removal is optimal. Preferably, the pH of the slurry is adjustedby the addition of nitric acid, potassium hydroxide, and/or ammoniumhydroxide.

Preferably, the slurry according to the present invention is prepared bydispersing the oxidizing agent in the solvent either before or after thenon-chelating free radical quencher has been added. If abrasiveparticles are to be included in the slurry, the slurry can be preparedin a number of ways. For example, the slurry may be prepared bydispersing the abrasive particles in a solvent either before or afterthe oxidizing agent and/or the non-chelating free radical quencher havebeen added. The slurry may also be prepared as a two-component system(i.e., an abrasive dispersed in deionized water component and anoxidizing agent and non-chelating free radical quencher in deionizedwater component). The slurry may also be prepared in concentrated formneeding only the addition of deionized water to dilute the concentrate(or concentrated components in a two-component system) to the desiredlevel.

Alternatively, the slurry according to the present invention may beformed by incorporating a portion of the components of the slurry in apolishing pad. For example, the abrasive particles and the non-chelatingfree radical quencher could be incorporated directly in the polishingpad, either with or without abrasive particles, and deionized water andthe oxidizing agent could then be added to the pad or the surface of thearticle being polished to form the polishing slurry in situ. In anotheralternative embodiment, the abrasive particles could be bonded to thepolishing pad, and the oxidizing agent, non-chelating free radicalquencher and deionized water could be added either separately ortogether to the pad or the surface of the article being polished to formthe polishing slurry in situ. It will be appreciated that the componentsof the slurry according to the invention could be combined in variousways to form the slurry in situ.

It is also possible to form the components of the slurry by combiningchemical precursors together either before or at the time of polishing.Thus, as used in the specification and in the appended claims, the term“slurry” should be understood to refer to the components present at theinterface between the polishing pad and the surface of the article beingpolished during chemical-mechanical polishing, and, unless otherwisestated, use of the term “slurry” is intended to encompass situationswhere precursors are combined to form the components of the slurry insitu.

The present invention is also directed to a method of removing copperoverlaying a tantalum-based barrier layer during the fabrication of acopper damascene structure having a plurality of copper lines. Themethod according to the invention comprises providing achemical-mechanical polishing slurry comprising an oxidizing agent thatreleases free radicals and a non-chelating free radical quencher that iseffective to retard the corrosion of said copper lines duringchemical-mechanical polishing, and polishing the copper layer using theslurry until the tantalum-based barrier layer is exposed.

The following examples are intended only to illustrate the invention andshould not be construed as imposing limitations upon the claims.

EXAMPLE

Slurry A was formed by dispersing 3.0% by weight alumina particleshaving an average particle diameter of 340 nm, 3.0% by weight hydrogenperoxide, and 1.0% by weight glycine in deionized water. Slurry B wasformed in the same manner and had the same composition as Slurry A,except that it also contained 1.0% by weight ascorbic acid. A sufficientamount of potassium hydroxide was then added to each slurry to adjustthe pH to 5.8.

Identical Sematech 926 patterned wafers (each wafer comprised a siliconsubstrate having an etched TEOS CVD silicon-dioxide dielectric layer3000 nm, a 25 nm Ta/TaN barrier layer, and 1600 nm copper layersequentially applied thereto) was separately polished with Slurries Aand B described above using a Strasbaugh 6CA polisher and a Rodel IC1400K-grooved pad. The polishing conditions were: 4 psi down pressure; 0 psiback pressure; 40 rpm table speed; 40 rpm quill speed; 20° C.temperature; and 200 cc/min slurry flow rate.

After the copper layer was removed to expose the Ta/TaN barrier layer bychemical-mechanical polishing, the surface of each wafer was examinedusing an optical microscope for copper line corrosion. The waferpolished using Slurry A showed severe copper line corrosion whereas thewafer polished using Slurry B, which contained a non-chelating freeradical quencher (i.e., 1.0% by weight ascorbic acid), showed no visiblecorrosion. The results of the foregoing Example are summarized in TableI below:

TABLE I Oxidi- Agent for Cu Slur- zing Abra- Quenching Other PolishingCu Line ry Agent sive Radicals Additives Rate Corrosion A 1% 3% NONE 1%900 Severe H₂O₂ Al₂O₃ glycine nm/min B 1% 3% 1% 1% 620 NONE H₂O₂ Al₂O₃ascorbic glycine nm/min acid

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and illustrative examples shown anddescribed herein. Accordingly, various modifications may be made withoutdeparting from the spirit or scope of the general inventive concept asdefined by the appended claims and their equivalents.

What is claimed:
 1. A method of removing copper overlaying atantalum-based barrier layer during the fabrication of a copperdamascene structure having a plurality of copper lines, said methodcomprising: providing a chemical-mechanical polishing slurry comprising:an oxidizing agent that releases free radicals; and a non-chelating freeradical quencher that retards the corrosion of said copper lines duringchemical mechanical polishing selected from the group consisting ofthiamine, 2-propanol, alkyl glycols, and combinations of the foregoing;and polishing said copper layer using said slurry until saidtantalum-based barrier layer is exposed.
 2. The method according toclaim 1 wherein said oxidizing agent that releases free radicals isselected from the group consisting of peroxides, peroxydiphosphates,persulfates, and combinations of the foregoing.
 3. The method accordingto claim 1 wherein said slurry has a pH of from about 4.0 to about 7.0.4. The method according to claim 1 wherein said slurry further comprisesabrasive particles.
 5. The method according to claim 4 wherein saidabrasive particles are selected from the group consisting of silica,aluminum, ceria, and combinations of the foregoing.
 6. The methodaccording to claim 4 wherein said abrasive particles comprise from about0.1% to about 60.0% of said slurry by weight.
 7. A method of removingcopper overlaying a tantalum-based barrier layer during the fabricationof a copper damascene structure having a plurality of copper lines, saidmethod comprising: providing a chemical-mechanical polishing slurrycomprising: hydrogen peroxide; and a non-chelating free radical quencherthat retards the corrosion of said copper lines during chemicalmechanical polishing selected from the group consisting of ascorbicacid, thiamine, 2-propanol, alkyl glycols, and combinations of theforegoing; and polishing said copper layer using said slurry until saidtantalum-based barrier layer is exposed.
 8. The method according toclaim 7 wherein said non-chelating free radical quencher comprises fromabout 0.01 % to about 5.0% of said slurry by weight.
 9. The methodaccording to claim 7 wherein said hydrogen peroxide comprises from about0.01% to about 15.0% of said slurry by weight.
 10. The method accordingto claim 7 wherein said slurry further comprises abrasive particles. 11.The method according to claim 10 wherein said abrasive particles areselected from the group consisting of silica, alumina, ceria, andcombinations of the foregoing.
 12. The method according to claim 10wherein said abrasive particles comprise from about 0.1% to about 60.0%of said slurry by weight.
 13. The method according to claim 7 whereinsaid slurry further comprises deionized water.
 14. The method accordingto claim 7 wherein said slurry further comprises pH adjusters and/or pHbuffers.
 15. The method according to claim 14 wherein the pH of theslurry has been adjusted to less than about 7.0.
 16. A method ofremoving copper overlaying a tantalum-based barrier layer during thefabrication of a copper damascene structure having a plurality of copperlines, said method comprising: providing a chemical-mechanical polishingslurry comprising: from about 3% to about 10% by weight of abrasiveparticles; from about 0.5% to about 5.0% by weight of hydrogen peroxide;and from about 0.1% to about 1% by weight of ascorbic acid; andpolishing said copper layer using said slurry until said tantalum-basedbarrier layer is exposed.
 17. The method according to claim 16 whereinsaid abrasive particles comprise alumina.
 18. The method according toclaim 17 wherein said slurry further comprises deionized water andglycine.
 19. The method according to claim 18 wherein the pH of saidslurry is within the range of from about 4.0 to about 7.0.